Single-photon sources [1]}, [2]} are key resources for quantum communication [3]}, photonic quantum computation [4]} or radiometry [5]}, [6]}. Not all single-photon sources are alike and to be scaled up, most applications require efficient sources of true single photons (single photon Fock/number states). The quality of single-photon sources is usually quantified from an auto-correlation measurement [7]}, that is, by using two photon detectors after a balanced beamsplitter and by checking that the ratio between the twofold coincidences and the product of singles vanishes. This ensures that the source produces no more than one photon. The result is however insensitive to loss as the efficiency cancels out of the ratio. These two aspects – the capacity of a source to produce no more than one photon and its efficiency – are thus considered separately. Both aspects are, however, important and are quantified jointly by the probability that the source actually produces exactly a single photon. Characterizing this probability is a direct and more complete way to benchmark single-photon sources.
<FIGURE>